It is rare, albeit less so all the time, for celebrities to open up about their personal battles with clinical depression. Older stars in particular tend to maintain a culture in which all mental health issues came with stigmas attached. Which is why it was unusual this week to see two stars of a more senior generation, Dick Cavett and Stephen Fry, openly discussing their struggles with depression on the Huffington Post.
Mr. Fry offered an illuminating assessment of what makes depression unique, namely that its agony, while invisible, may still lead to suicide, even in busy people:

Fry emphasized how seemingly-contradictory the messages we send about depression can be. "Okay, you've got this problem, it doesn't stop you from being a high-functioning individual," he said, and yet on the other hand, it can cause death. It's not to be taken too lightly, he said, and at the same time, it's not to be taken as a death sentence.

Of course this is the point: depression doesn’t have to paralyze you to impair your life in significant ways. The classic image of the depressive who can’t get out of bed in the morning is simply one face this problem wears; other symptoms may include quiet moments of despair, chronic sleeplessness, broken relationships and diminished productivity.

Depression can be caused by genetics, chemistry or psychology. The only way to know for sure is to try analytic therapy and explore whether your issues may are emotionally based, or simply biological in nature.